It turns out that <style> tags don't work, so it's a no-go on my original plan to define an .erogame{...} class style and then just use <p class="erogame"> throughout the thread.
Instead, here's an inline style definition for Erogame violet-pink text boxes.
(The general principle should be applicable to other situations as well.)
Erogamer's CSS: Lvl 1 (0%). Active. 1 TEC / use.
You can create violet-pink text boxes floating in midair.
Irresponsible use of this power may lead to shenanigans, so be sure to wield it carefull--+so there's literally no downside. Go wild.
At Lvl 1, text boxes too close to the top of a post or reply will clip incorrectly against the userbox on the left side. You can avoid this by adding dialogue or narration above the game's text box. Use of the preview feature is recommended.
Quest available: It Just Bugs Me
Submit a code patch to let the Erogame text box appear correctly even when placed at or near the top of the post or reply.
Hint: This issue also affects normal blockquotes.
Success: +1 Erogamer's CSS, increased relations with users
Failure: ---
Bonus objective #1: Make sure the top of the text box doesn't clip the bottom of the userbox when applicable.
Accept: Y/N
Here's the actual code:
<blockquote style="
color: #800080;
border: 0.25em solid #800080;
background: black;
font-family: monospace;
padding: 1em;
border-radius: 1em;
">Game text goes here.</blockquote>
Here's a step-by-step on how to use this.
1. Surrounding text
If you just paste the code on its own, you won't be able later to easily add text outside of the specially-formatted box; there needs to already be outside text in place. (If you skip this step, the situation is recoverable but takes a tiny bit of hand-editing in HTML mode.)
Start in Rich Text mode. (If you don't know what this is, you're almost certainly in it by default.)
Add a couple lines of placeholder text. (Two is sufficient: one will end up above the special game-info box, and the other will end up below.)
After you've done this, click the "HTML" button to switch the post editor into HTML mode.
2. Paste the magic style code
Copy the code block near the top of this instruction post (the one starting with <blockquote style=" and ending with </blockquote> ) and paste it between the two lines.
After you've done this, click the "Rich Text" button to switch out of HTML mode and back to normal editing.
3. Write your content
You should now have the specially-formatted Erogame text box displaying inline and editable in the post editor.
By default, pressing Enter will create a separate text box immediately after the current one. (Sorry.) To insert newlines inside a text box, use Shift-Enter.
Final note: In-universe, the background should be transparent rather than opaque black, but that's hard to read if the reader's preferred site skin uses a mediumish (as opposed to very dark or very light) background color for posts.